Photographically useful agents that are incorporated in a photographic light-sensitive material such that their effects will be produced at the appropriate time contain various features different from those which are used as an addition to a processing solution. Specific examples of such features in the former case are as follows. First, the incorporation in a photographic light-sensitive material enables effective utilization of photographic agents of kinds which tend to decompose under the acidic, alkaline, or oxidation-reduction conditions, and consequently, cannot withstand long time storage in a processing bath. At the same time, such makes it possible to simplify the composition of the processing solution to be employed therewith, and thereby to facilitate the preparation of the processing solution. Further, this makes it possible to force a required photographic agent to function at a desired time during the photographic processing, or only at a desired place, that is, in only a specified layer and the neighboring layers of a multilayer photographic light-sensitive material. Furthermore, this permits the presence of a photographic agent in the photographic light-sensitive material in such an amount as to vary as the function of silver halide development. However, if a photographic agent is added to a photographic light-sensitive material in its active form, it becomes impossible to make the photographic agent exhibit its ability to the expected degree, because during storage before photographic processing, it reacts with other components contained in the photographic light-sensitive material, or it is decomposed by heat, oxygen, etc.
One method for solving the above-described problem involves adding a photographic agent to a photographic light-sensitive material in such a form that its active group is blocked and turned photographically inactive, that is, it is present in the form of a precursor. Such a method can have various advantages in various cases to which it is applicable. For instance, in the case where the useful photographic agent is a dye, blocking a functional group of the dye, which has a great effect on its spectral absorption characteristic, results in a shift of its spectral absorption band to shorter wavelengths or to longer wavelengths and therefore, even if the dye is present in a silver halide emulsion layer having the spectral sensitivity in the wavelength region corresponding to the absorption band which the dye has in the unblocked state, a lowering of the sensitivity due to the so-called filter effect can be prevented.
In another case, wherein the useful photographic agent is an antifoggant or a development restrainer, blocking of the active groups makes it possible to suppress desensitization arising from adsorption of these agents to light-sensitive silver halide grains and formation of silver salts upon storage. At the same time, release of these agents at required times permits the reduction of fog density without being attended by a decrease in the sensitivity, the prevention of fog due to over-development, development stoppage at a desired time, and so on.
In still another case, wherein the useful photographic agent is a developing agent, an auxiliary developing agent, or a fogging agent, if their active or adsorptive groups are blocked, various photographically adverse effects which arise from semiquinones and oxidants produced by air oxidation upon storage can be prevented. Generation of fogging nuclei upon storage can also be prevented because injection of electrons into the silver halide grains can be inhibited. Therefore, stable processings can be effected therein.
In a further case, viz., that the useful photographic agent is a bleach accelerating agent or a bleach-fix accelerating agent, it also becomes possible to prevent reactions with other components also present in the photographic light-sensitive material from occurring upon storage by blocking its active group, and that, to make its expected ability bring into full play at a desired time by removing the blocking group.
As described above, a precursor of photographic agents can be utilized as an extremely valuable tool in bringing out the abilities of the photographic agents to best advantage. However, the precursor must satisfy very severe requirements for practical use. That is, the precursor must satisfy two requirements that are somewhat contradictory to each other; one is ensuring stable presence of the precursor under a storage condition, and the other is setting its blocking group loose at a desired time during the processing and releasing the photographic agent rapidly and efficiently.
A number of techniques for blocking a photographic agent are already known. For instance, a technique using a blocking group such as an acyl group, a sulfonyl group or the like is described in U.S. Pat. No. 3,615,617; a technique which utilizes such a blocking groups as to release a photographic agent by the so-called reversal Michael reaction is described in U.S. Pat. Nos. 3,674,478, 3,791,830 and 4,009,029; a technique which utilizes such a blocking group as to release a photographic agent with the production of quinone methide or its analogs by intramolecular electron transfer is described in U.S. Pat. Nos. 3,674,478, 4,416,977 and 4,420,554; a technique which utilizes an intramolecular ring-closing reaction is described in U.S. Pat. No. 4,310,612; a technique which utilizes cleavage of a 5-membered or 6-membered ring is described in U.S. Pat. Nos. 4,335,200 and 4,350,752 and Japanese Patent Application (OPI) 179842/82 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"); and so on. However, these photographic agents blocked with known blocking groups suffer from certain defects; for example, although stable under storage conditions, some precursors require a highly alkaline condition, such as a pH higher than 12, for processing because the photographic agent-releasing rate thereof is too slow; some precursors decompose gradually losing their function as the precursor under storage conditions, even though it can release the photographic agent at a sufficiently fast rate by processing under mild conditions such as at a pH in the range of 9 to 12.